cmd/upspin: improve documentation for the user subcommand
cmd/upspin: improve documentation for the user subcommand

The -put flag was not well explained. Also add a handy usage tip
(upspin user | upspin user -put).

Fixes #242

Change-Id: I022db47b3db5532c30a7c6c3c546bdc44250731f
Reviewed-on: https://upspin-review.googlesource.com/7977
Reviewed-by: adg <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>

upspin/upspin,upspin/upspin